It is conventionally proposed to increase the impurity concentration of an n-barrier layer between an n-base layer and a p-base layer so as to decrease an ON-state voltage of an IGBT (Insulated Gate Bipolar Transistor). When the impurity concentration of the n-barrier layer is increased, then a potential barrier of the n-barrier layer for holes increases, and the number of holes accumulated in the n-base layer increases, resulting in the decrease in the ON-state voltage of the IGBT.
However, the conventional technique has the following problems. When the impurity concentration of the n-barrier layer is increased, more holes are attracted to an electron current along a MOS channel and accumulated around a trench gate. When more holes are accumulated around the trench gate, negative charge is induced in the trench gate, and a variation slope (dQg/dVg) of between a gate charges (Qg) and a gate voltage (Vg) is decreased to a negative vale (to negative capacitance). When the variation slope is negative, then a voltage applied to the trench gate varies largely, the potential of a gate electrode rises, and a current excessively is applied to the IGBT. As a result, the IGBT may be broken down in.